1. Field of the Invention
The present invention relates to a power supply apparatus which outputs a high-tension alternating voltage used in a copying machine, a printer or the like to perform image formation in an electrophotographic method, a control method for the power supply apparatus, and an image formation apparatus which contains the power supply apparatus.
2. Related Background Art
In general, a high-tension alternating voltage might be used in an image formation apparatus such as a copying machine or the like which performs image formation in an electrophotographic method.
FIG. 5 is a circuit diagram showing a structure of a power supply device (a high-tension output device) used in a conventional image formation apparatus. In order to generate the above high-tension alternating voltage, as shown in FIG. 5, a center-tap converter transformer in which a primary winding 302a has a center tap is sometimes used.
In FIG. 5, numeral 301 denotes a converter transformer driving circuit, and numeral 302 denotes a center-tap converter transformer (simply referred as a transformer hereinafter). Numeral 303 denotes an output terminal which is connected to a not-shown load used for the image formation. Each of symbols Tr1 and Tr2 denotes an N-channel MOSFET (metal oxide semiconductor field-effect transistor: simply referred as a transistor hereinafter).
The transistors Tr1 and Tr2 are connected to the transformer 302. When the transistors Tr1 and Tr2 are on-off controlled, power is generated at a secondary side of the transformer 302 and then supplied to each load through the output terminal 303.
When control signals (oscillation signals) IN1 and IN2 shown in FIG. 6 are input respectively to gates of the transistors Tr1 and Tr2, these transistors Tr1 and Tr2 perform switching at on/off timing as shown as waveforms of the control signals IN1 and IN2. Thus, when the transistors Tr1 and Tr2 alternately perform the switching, the transformer 302 is excited, and a high-tension alternating voltage according to a turns ratio is generated on the side of a secondary winding 302b. The generated voltage is output from the output terminal 303 (a waveform OUT in FIG. 6) to the load.
However, in the above conventional power supply device (the high-tension output device) which uses the center-tap converter transformer, when both the control signals IN1 and IN2 are off, the output oscillation is stopped. Thus, as shown at a part "C" of FIG. 6, at the instant that an operation state changes from a driving state to a stop state, an overshoot or a resonant voltage may occur in an output voltage. This output voltage changes due to the load or floating capacity, thereby generating unstable output voltage.
Thus, there is some fear that this output voltage influences the image formation and thus invites image quality deterioration, whereby there is a problem that a stability of the image quality decreases.